Why has this sensor not been made?

The D800 sports a FF version of the D7000 sensor, which is why it's so good. How about this? A FF version of the G12 sensor?

It would have 100 MP, a 55% QE, per-pixel read noise of 4.3 electrons (throughout the ISO range, i.e., ISOless), and a per-pixel saturation of 10306 electrons.

9.5 of these pixels would cover the same area a 5D3 pixel (which has a QE of 49%). Thus, the combined read noise for 9.5 pixels would be 13.2 electrons, as compared the the 5D3 pixel which has a read noise of 33.1 electrons at base ISO to a low of 2.4 electrons at ISO 12800), and a combined saturation of 97692 electrons compared to 67531 for the 5D3.

This would put the DR at 12.9 stops at base ISO (compared to 11.0 stops for the 5D3) down to 7.5 stops at ISO 12800 (compared to 7.8 stops for the 5D3).